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Date: Sat, 3 May 2008 00:59:34 +0100
Reply-To: RQuadling@GoogleMail.com
To: "Alain Williams" <addw@phcomp.co.uk>
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Subject: Re: [PHP-DEV] Float comparison
From: rquadling@googlemail.com ("Richard Quadling")

2008/5/3 Alain Williams <addw@phcomp.co.uk>:
> On Fri, May 02, 2008 at 01:03:45PM -0700, Todd Ruth wrote:
>  > On Fri, 2008-05-02 at 11:41 -0700, Rasmus Lerdorf wrote:
>  > > Stefan Walk wrote:
>  > > > And you'll quickly see that the "cast to string before comparision" is a
>  > > > bad idea, because:
>  > > > $ php -dprecision=1 -r 'var_dump((string)1.4 == (string)1.1);'
>  > > > bool(true)
>  > > >
>  > > > Having display settings affect comparisions seems like a really bad idea
>  > > > to me ;)
>  > >
>  > > Yup, it would be a fatal flaw in the language if that ever came to be.
>  > >
>  > > -Rasmus
>  >
>  > Most people don't care about floating numbers beyond a certain
>  > number of digits.  If precision were the only ini setting that
>  > gave us an idea of how much the person cared, I think it would
>  > be fair to use the setting or introduce a new setting.  As it
>  > happens, there is another setting.  How would you feel about
>  > this returning true:
>  >
>  > php -dserialize_precision=1 -r 'var_dump(1.4 == 1.1);'
>  >
>  > Setting serialize_precision to 1 is a very strong statement of
>  > not caring about those digits.  Personally, I don't care about
>  > more than about 6 digits and I don't do much math, so I could
>  > set it to 8 digits and be happy.  Not everyone has the luxury
>  > of controlling ini settings, but for those that do, this
>  > would be much nicer than having
>
>  Hmmm, what you are talking about is implying:
>
>         $a == $b
>
>  be implemented as:
>
>         abs($a - $b) < (max(abs($a), abs($b)) / EPSILON)
>
>  Where EPSILON is some sort of accuracy factor. It might be
>  possible to implement this easier/faster at a machine code level.
>
>  The trouble is that doing something like this would be global and
>  might mess up some code written elsewhere. Mind you: any programmer
>  going '$a == $b' on true floating point has got to expect rubbish.
>
>  This is making things easier for the naive/novice programmer. In this case
>  I don't think that it realy helps since the naive programmer isn't
>  going to have a clue about choosing a good value for EPSILON.
>
>  Best leave it the way that it is.

What about introducing a "money" or "currency" numeric type which
guarantees say accuracy to 8DP?




-- 
-----
Richard Quadling
Zend Certified Engineer : http://zend.com/zce.php?c=ZEND002498&r=213474731
"Standing on the shoulders of some very clever giants!"